1. Field of the Invention
This invention relates to a novel process for removing a non-aqueous solvent from a substrate and thereafter recovering said solvent. In another aspect, this invention relates to a process for removing a non-aqueous solvent from coated or impregnated paper and a novel apparatus therefor.
2. Description of the Prior Art
The quest for a more efficient and effective process for removing non-aqueous solvents, e.g., organic solvents, from a substrate such as paper, cloth and non-woven or woven fabrics has been continuous. This is particularly true for areas of technology employing commercial processes involving the impregnation of a substrate with a resin wherein the resin is applied as a solution. The solution generally comprises a non-aqueous, organic solvent, removal and recovery of which is desirable for economic and environmental reasons.
The paper industry is one specific industry where the need for a more efficient and effective process for the removal of non-aqueous solvents is pronounced. More particularly, such need is especially pronounced in that portion of the paper industry pertaining to the impregnation of a paper substrate with phenol-formaldehyde resin. The phenol-formaldehyde resin is generally applied in an alcohol, e.g., methanol solution, with the methanol solvent being removed in a hot air oven. (Compare U.S. Pat. No. 2,991,194 issued to Cambron, July 4, 1961.)
The use of a hot air oven, however, has many disadvantages. For example, there is danger of combustion due to the presence of oxygen. To reduce this danger, the volume of air that is heated and employed is generally very large in order to provide sufficient dilution of the organic solvent to keep the concentration of the alcohol below a predetermined hazardous level.
Other disadvantages lie in the cost and the difficulty of recovering the alcohol solvent. Once the hot air has removed the solvent, the recovery of the alcohol from the hot air requires cooling the air and passing the cooled gas through a bed of activated charcoal on which the alcohol is adsorbed. The charcoal bed is then stripped of solvent by using steam, the steam-solvent vapor mixture is condensed, and finally, the steam-solvent mixture is distilled to recover water and solvent. However, because of the difficulties and expense involved in recovering the alcohol solvent by the foregoing, and other known methods of recovery, the alcohol vapors originating from the resin are generally exhausted to the atmosphere, or incinerated without recovery, in the hot air oven exhaust gas. This can produce a severe environmental impact.
Another disadvantage of using hot air to remove the organic solvent is that the presence of the oxygen, particularly in such large quantities, presents a favorable environment for the oxidation of the product. Also, the use of hot air requires heating large quantities of air, which can be wasteful and inefficient from an energy standpoint.
Other workers have employed steam as a heat source for evaporating organic solvents from substrates, but the heat supplied by the steam has always comprised some latent heat thereby resulting in the condensation of steam on the substrate. (Compare U.S. Pat. No. 1,261,005 issued to Barstow et al; U.S. Pat. No. 2,174,170 issued to Schweizer; U.S. Pat. No. 2,565,152 issued to Wachter et al; U.S. Pat. No. 2,590,850 issued to Dungler; and, U.S. Pat. No. 3,089,250 issued to Victor.)
Problems can arise with the condensation of steam on the substrate, however, in that the condensation can be very deleterious to a quality product, particularly if the substrate is a water sensitive substrate such as paper. For example, phenolic coated paper preferably has a moisture content of up to about 6% or less. If the moisture content increases much above 6%, e.g., to 10%, the paper will be too wet and will suffer in such properties as stiffness. Uncontrolled condensation of steam on a substrate, therefore, can be very detrimental to a product's properties.
In two other patents, U.S. Pat. No. 2,760,410 issued to Gillis and U.S. Pat. No. 3,761,977 issued to Rappoport, the use of steam for the removal of water is disclosed. In the Gillis patent, dry superheated steam is employed at pressures up to 60 psi to evaporate the water from newly formed paper. In the Rappoport patent, superheated steam at pressures in excess of 5 psig, and up to 115 psig, is employed to remove water from textile materials by directing the steam through the textile material. Neither of the patents, however, suggest or disclose a method for effectively and efficiently removing non-aqueous solvents from a substrate which avoid the aforementioned problems.